Red blood cells (RBC), or erythrocytes, are responsible for the transport of oxygen throughout the body. Specifically, hemoglobin, which is part of a healthy red blood cell, is an iron-containing respiratory macromolecule which functions in its oxygenated form to carry oxygen from the lungs to tissue sites. As is known, red blood cells have a membrane that is freely permeable to glucose. When glucose or other sugar moieties enters a red blood cell, glycated hemoglobin (GHb) can be formed. Glycated hemoglobin refers to various hemoglobin derivatives formed by covalent attachment of sugar moieties, in particular glucose. The amount of glycated hemoglobin formed is related to glucose concentration in the blood, as well as to the duration of exposure to glucose. The common index of glycated hemoglobin is known as HbA1c, and the ratio of HbA1c to total hemoglobin is known as % HbA1c.
As a general matter, red blood cells have a lifetime of approximately 120 days, and the amount of glycation of hemoglobin varies as a function of the 120 day lifetime of the cell. As the reaction of glucose with hemoglobin molecules is slow and generally irreversible in vivo, the amount of glycated hemoglobin (HbA1c) has traditionally been considered to be an accurate index of blood glucose concentrations over the previous 3-4 months. Accordingly, diabetes patients are advised to check their HbA1c value every 3-4 months. For example, the American Diabetes Association (ADA) recommends an HbA1c test about 2 to 4 times per year and further recommends an HbA1c level below 7%.
The determination of total hemoglobin can be performed by simple absorbance or reflectance measurements. Typically the iron is reduced by potassium ferricyanide to create methemoglobin, which is measured at 565 nm. The determination of the amount of glycated hemoglobin can be performed by numerous methods known in the art such as by competitive or immunometric assays. In the latter, the antigens can be insolubilized on a solid phase and a labeled antibody is incubated in their presence. The antibody-antigen complex can be detected by optical or fluorescent methods. This method can be practiced on about 10 μl of blood. However, this method is carried out using a random cell sample collected from the blood, and thus, provides a single datum of average glucose regulation history at a specific point in time. The measured average value can be skewed toward newer cells because they are more prevalent. Yet the measured average amount of glycation of hemoglobin is often understated for younger cells because of their brief exposure to glucose, when compared to older cells. Because of this, testing glycated hemoglobin every 3-4 months will not accurately reflect the quality of glucose regulation in some diabetic patients. For instance, a diabetic patient can be systematically under-regulating their glucose levels for 1-3 months, and then over-regulating their glucose levels during the most recent 30 days prior to the test. Because of the large influence that the newest red blood cells have on HbA1c testing, the average glycated hemoglobin can appear in the normal range, thus misleading the patient and the caregiver as to the quality of glucose regulation. As a result, it would be beneficial to provide a practitioner or a subject more detailed information related to patient glucose levels over shorter periods of time, as well as provide more accurate information related to compliance with respect to glucose regulation.